Layered silicates typified by clay minerals and mica minerals are classified in detail on the basis of the constituent elements and the layer charges thereof. The basic structure of the layers of layered silicate is primarily composed of a tetrahedral sheet and an octahedral sheet. In the tetrahedral sheet, tetrahedrons in which four O2− coordinate with a metal, e.g., silicon or aluminum, are bonded into a hexagonal network shape so as to form a sheet. In the octahedral sheet, octahedrons in which six OH− or O2− coordinate with a trivalent, divalent, or monovalent metal, e.g., aluminum, magnesium, or lithium, are joined sharing edges. This tetragonal sheet and the octagonal sheet are joined sharing apex oxygen. A layer in which one tetrahedral sheet is bonded to one octahedral sheet is referred to as a 1:1 layer, and a layer in which tetrahedral sheets are bonded to both sides of one octahedral sheet is referred to as a 2:1 layer.
Layered silicates having a 2:1 layer are classified on the basis of the magnitude of layer charge. Furthermore, a layered silicate in which divalent cations, e.g., Mg2+, enter all site of the octahedral sheet can be classified into di-octahedral type, and a layered silicate in which trivalent cations, Al3+, enter two sites and one site is vacant can be classified into tri-octahedral type. Regarding each of them, if a shortage of positive charge occurs in the octahedral sheet or the tetrahedral sheet because of substitution with metals having different valences, the charge of the entire layer becomes negative. An absolute value of charge (negative) of a layer with reference to an ideal chemical composition is referred to as a layer charge. Therefore, when a layer charge occurs, cations are required between the layers in order to keep a charge balance.
The layered silicates having 2:1 layers exhibiting layer charges within the range of 0.2 to 0.6 are referred to as smectite and there are di-octahedral smectite and tri-octahedral smectite. Montmorillonite, beidellite, nontronite, and the like are known as the di-octahedral smectite. Saponite, hectorite, stevensite, and the like are known as the tri-octahedral smectite. Smectite is a fine particle clay mineral, and has specific properties, e.g., the ion exchange ability manifested from the layer charge, the swellability, the dispersibility, and the intercalation function.
Examples of layered silicates having 2:1 layers exhibiting layer charges within the range of 0.6 to 1.0 include vermiculites; mica clay minerals typified by illite, sericite, glauconite, celadonite, and the like; and mica minerals typified by phlogopite, biotite, muscovite, palagonite, and the like. The crystallinity in directions of the a axis and the b axis is high and sheet area of each sheet is large as compared with a smectite crystal. However, nonexchangeable potassium ions are often included between the layers, and the swellability with water and the like are not exhibited in contrast to smectite.
Regarding mica clay minerals and mica minerals, in lamination of silicate layers, there are some azimuths of hydrogen bonds between tetragonal sheets and octagonal sheets and azimuths of displacement of tetragonal sheets sandwiching octagonal sheets, wherein the energy levels are equal but the crystalline structures are geometrically different depending on the azimuths. Therefore, three-dimensionally different crystalline structures may result depending on the azimuth of lamination chosen from possible azimuths. A different type of manner in regular lamination occurs on the basis of the difference in manner of stacking of the layers. This is referred to as a polytype (for example, Non-Patent Document 1).
On the other hand, examples of layered silicates having 2:1 layers which do not manifest a layer charge structurally include tri-octahedral talc and di-octahedral pyrophillite. They do not exhibit cation exchangeability. They do not swell with nor disperse in water in contrast to smectite. Talc is a tabular crystal, and is industrially used as fillers for polymeric materials and the like.
Regarding layered silicates having 1:1 layers, examples of di-octahedral type can include kaolin minerals of kaolinite, dickite, nacrite, and halloysite and examples of tri-octahedral type can include serpentine minerals, e.g., chrysotile and lizardite. These do not exhibit a layer charge. In kaolinite and dickite, 1:1 layers are laminated while being shifted by −a/3 from each other in the a axis direction of the crystal. At this time, the polytype occurs depending on the distribution of vacant positions of cations in the octahedrons resulting from lamination. Many polytypes occur regarding nacrite, chrysotile, and lizardite as well.
These layered silicates have been used previously in wide fields, e.g., potteries, refractories, castings, civil engineering, petroleum refining (catalyst), paper, medicines, cosmetics, and fillers for plastic. Recently, attempts to control the structure at a nanometer level and apply to functional materials have been conducted actively. In particular, regarding swellable layered silicates, e.g., smectite, the two-dimensionality, the layer charge, the ion exchangeability, and the self-assembly based on the interaction with organic molecules of them are used well and the layered silicates are used for special, industrial purposes, e.g., intermediate compounds and polymer-clay nanocomposites.
For example, various cationic coloring agents are intercalated into smectite, and the resulting organic-inorganic complexes can be used as color couplers of thermal transfer color printers taking advantage of the color development action thereof (Patent Document 1). An inorganic nanosheet-dispersion complex system of nanometer level is obtained by combining organic-inorganic complex, in which an organic cation is intercalated into smectite or swellable fluorine mica, with a polymer, so that a layered silicate-polymeric nanocomposite having dramatically improved heat resistance, rigidity, and gas barrier property of the material can be obtained (Non-Patent Document 3). The application of the nanocomposite has been developed in various purposes.
In general, among the above-described layered silicates, smectite is one of clay minerals and it is easy to intercalate organic molecules between layers thereof so as to form an organic-inorganic complex. The smectite exhibits the swellability and has exchangeable cations between layers and, therefore, can be combined with various organic molecules and polymeric materials. However, the smectite is the smallest inorganic fine particle polycrystal in the soil, and the primary particle diameter is specified to be 2.0 μm or less (refer to “Nendo Handobukku (Clay Handbook)” second edition, edited by the Clay Science Society of Japan, Section 1 to Section 6, GIHODO SHUPPAN), and the lamination thereof exhibits significant irregularity.
In the X-ray diffraction, only broad two-dimensional reflection (hk band) is observed except the bottom surface reflection. Therefore, the smectite is believed to be a two-dimensional crystal not having a three-dimensional structure due to regular lamination. The synthetic fluorine mica exhibiting swellability with water and the like similarly to the smectite has crystallinity of the layer surface in the a axis and the b axis directions higher than that of the smectite, and the sheet area per sheet is larger. However, a fine particle crystal results as compared with mica minerals, mica clay minerals, vermiculites, talc, pyrophillite, kaolin minerals, and the like.
The swellable layered silicates, e.g., smectite and swellable fluorine mica, easily form organic-inorganic complexes by an ion exchange reaction. However, among the above-described non-swellable layered silicates, for example, regarding mica clay minerals and mica minerals, potassium ions fit into an oxygen six-membered ring on the two, i.e. upper and lower, silicate sheet surfaces, and the non-swellability is induced on the basis of the electrostatic cross-linking effect thereof (Non-Patent Document 2).    Non-Patent Document 1: Crystalline structures of clay minerals and their X-ray identification, Edited by G. W. Brindley and G. Brown, Mineralogical Society, London, 1980.    Non-Patent Document 2: H. van Olphen, Chap. 5 Clay Mineralogy, III.ILLITE (NONEXPANDIND 2:1 LAYER CLAYS), In “An Introduction to Clay Colloid Chemistry: for clay technologists, geologists, and soil scientists” 2nd ed, New York: Wiley, (1977) 68-69.    Non-Patent Document 3: Polymer-Clay Nanocomposites, 1st ed, Edited by Pinnavaia, T. J.; Beall, G. W.; Wiley Series in Polymer Science; Wiley: New York, 2000.    Patent Document 1: Japanese Patent No. 2770409